Sector number should be generated by the Storage Provider.

[jmg-duarte]

          Sector number can (and should) already be generated by the Storage Provider.

It is accepted by Storage Provider Pallet and it's internal numbering of the Storage Provider Node.
When we're doing with integrating the PoRep, we'll take sector number from Sector Data stored on chain and use it to prove.
This is not blocked, and I don't want you to do this in this PR, but this can be a certainly a next task to do.

Originally posted by @th7nder in #470 (comment)